512 | JULY 2004 | VOLUME 5 www.nature.com/reviews/molcellbio

R E S E A R C H H I G H L I G H T S

Many bacteria show polar character-istics, such as the positioning of aflagellum at one end of the cell.However, the mechanisms involvedin establishing this polarity are poorlyunderstood. A recent study fromLucy Shapiro and colleagues hasmade important progress in under-standing these processes by identify-ing a master regulator of polarity inCaulobacter crescentus.

The C. crescentus life cycle involvestwo different cell types, both withspecialized structures located at onepole of the cell. Several proteins thatare involved in the development ofthese structures and that have corre-sponding polar distributions havebeen identified, providing usefulmarkers of polarity. Shapiro and col-leagues made use of these features toinvestigate whether the actin-likeprotein MreB is required for polar-ization in C. crescentus.

MreB has a distinctive localiza-tion pattern, forming a spiral struc-ture that extends along the length of

C. crescentus cells. By analogy toeukaryotic actin, MreB moleculesmight have an intrinsic polarity, sothe spirals they form could be usedfor the asymmetric localization ofmolecules required for the develop-ment of polar structures. To test this,the authors analysed the effect ofMreB depletion on the distributionof four signalling proteins — PleC,DivJ, CckA and DivK — that arerequired for polar development inC. crescentus. Depletion of MreBabolished the polar foci that are usu-ally formed by all four proteins at cer-tain points in the cell cycle, consistentwith a role for MreB as a global regu-lator of polarity.

Importantly, MreB seems to beactively required for specifying polar-ity, rather than having a passive rolein protein localization. Unlike CckAand DivK, which form foci at bothpoles of C. crescentus cells, PleC andDivJ are asymmetrically distributedat certain points in the cell cycle,localizing to only one pole. When

A new report in Science describes the rapididentification and characterization ofproteins that function in cytokinesis, usingan approach that combines functional-proteomic and comparative-genomicanalysis.

From synchronized Chinese hamster ovary(CHO) cells, Ahna Skop and colleaguesisolated midbodies — microtubule-rich,transient structures that are derived from thespindle midzone and that exist after celldivision, just before the daughter cells detach.The proteins in the midbody preparationwere identified by a protein identificationtechnology — MudPIT — that is used for theanalysis of proteins in complex proteinmixtures.

After eliminating proteins that havegeneral housekeeping functions, 160candidate midbody proteins remained, ofwhich 57 were known cytokinesis proteins.Most candidate midbody proteins seemed tobe conserved in evolution as 147 out of 160proteins had clear Caenorhabditis elegans

homologues. The functional relevance of thecandidate midbody proteins becameapparent when the authors analysed genefunction by RNA interference using double-stranded (ds)RNA that corresponded to eachof the C. elegans genes. Most of thesedsRNAs produced a disrupted cytokinesisphenotype.

By analysing the various mutantphenotypes, the authors found that asignificant percentage of midbody proteinsthat function in cytokinesis also producedefects in germline development when theyare disrupted. They went on to show thatgonad development and sterility mutantphenotypes are, in fact, frequently caused bydefects in cytokinesis in the germline or inearly embryos. In addition, 16 proteins thatare essential for embryo and germlinecytokinesis were also required for polar-bodyextrusion (the polarized, asymmetric celldivision that occurs during meiosis). So,cytokinesis, gonad organization and polar-body extrusion seem to use a common set ofproteins.

Although membrane and cytoskeletalproteins are known to be involved incleavage-furrow formation, among othercytokinesis processes, how these proteins arerecruited to the cleavage plane has beenunclear. Skop and co-workers identified 40membrane and cytoskeletal proteins, which

include proteins that are involved in lipid-raft formation and vesicle trafficking. Theysuggest that raft-associated factors couldtarget and activate specific membrane eventsin cytokinesis.

Among the midbody proteins, 24% wereGolgi-associated proteins, which led theauthors to speculate that there might be someparallels between cytokinesis in animals andplants, as Golgi-derived vesicles are involvedin cell-wall formation after cell division inplants.

Finally, the functional analysis ofmidbody proteins revealed someunexpected phenotypes — 20% of theidentified proteins caused defects incoordination, which implicates theseproteins in muscle or neuronaldevelopment. In addition, 14% of themammalian proteins that were identifiedare known to have a role in human diseases,mostly those that are associated withmembrane and cytoskeletal pathologies. Sothis study provides plenty of starting pointsfor further investigations — including intocytokinesis, development or diseasemechanisms.

Arianne HeinrichsReferences and links

ORIGINAL RESEARCH PAPER Skop, A. R. et al.Dissection of the mammalian midbody proteome revealsconserved cytokinesis mechanisms. Science 27 May 2004(doi:10.1126/science.1097931)

A good place to start

C Y TO K I N E S I S

Polar exploration

C E L L P O L A R I T Y